Ectoparasite diversity and Orientia tsutsugamushi infection in rodent hosts from scrub typhus-endemic areas of the Western Himalayas, India.

Scrub typhus has re-emerged as an important public health concern in Himachal Pradesh, India, and is caused by Orientia tsutsugamushi. Effective control efforts are limited by outdated data on ectoparasite vectors and infection rates among reservoir hosts in this endemic region of the Western Himalayas. We conducted a comprehensive field study across eight districts of Himachal Pradesh to examine ectoparasite diversity associated with rodents and shrews in an area endemic for scrub typhus. Small mammals such as rodents and shrews were trapped, and their ectoparasites were collected and morphologically identified. Host infection was screened by PCR targeting the 56-kDa type-specific antigen (TSA) gene and by indirect ELISA. Among the 96 small mammals tested, ectoparasite infestations were observed in 35.4%. A total of 1033 ectoparasites, predominantly chigger mites (n = 851), were collected. We documented a trap positivity rate (TPR) of 21.9% [95% 17.9-26.5], a rodent infestation rate (RIR) of 33.3% [95%CI:24.9-43.1], and a chigger infestation rate of 25.0. Both TPR (P < 0.001) and RIR (P = 0.019) significantly varied between the districts. Morphological identification of 349 chiggers revealed about 30 species, including 15 new records for Himachal Pradesh and one new record for India. PCR detected O. tsutsugamushi DNA in 38.5% (37/96) of host samples, with district prevalence ranging from 0% to 100%. Serology confirmed widespread exposure. This study reports a high diversity of ectoparasites and a significant presence of Orientia tsutsugamushi in small mammals in Himachal Pradesh. The results confirm active enzootic transmission cycles and highlight the importance of integrated one-health surveillance of hosts and vectors to guide scrub typhus control efforts.